DNA repair was studied by the BUdR-CsCl centrifugation technique in primary cultures of mouse skin cells treated with the carcinogen MNNG. When both BUdR and a tritiated DNA precursor are present during DNA repair synthesis, tritium incorporated into light density, unreplicated DNA indicates DNA repair. Repair was shown with all deoxyribonucleoside precursors tested after treatment for 1 hr with high, toxic doses of MNNG (25-50 micrograms/ml). However, at lower doses of MNNG (1-5 micrograms/ml), repair synthesis was detected only with GdR3H. Paper chromatography of enzymatic hydrolysates of GdR-3H repaired DNA indicated that 99.5% of the tritium was incorporated into DNA guanine. Doses of 1, 5 or 25 micrograms/ml MNNG reduced DNA synthesis at 1-2 hr by 19%, 62% and 96% and decreased the amount of DNA per dish at 26 hr by 12%, 32% and 69% respectively, thus indicating significant toxicity with the highest dose. DNA repair synthesis was examined with TdR and GdR at 2 or more doses of MNNG in mouse epidermal cells, mouse fibroblasts, rat mixed cultures, rat epidermal cells and WI-38 human fibroblasts. GdR- specific repair was found in all rodent cells examined, but in WI-38 cells repair was demonstrated equally well with both precursors at all MNNG doses.